1. Field of the Invention
This invention is concerned with novel lubricating oil antioxidants, particularly those comprising a synergistic combination of dithioketals and phenolic or phosphite ester antioxidants.
2. Discussion of Related Art
A desired property of lubricating oils is its ability to withstand the effects of oxidation. Among the ill effects of lubricating oil oxidation are increases in the viscosity of the lubricant, development of acidic contaminants, and the formation of carbonaceous matter. Thus, there is a need for compositions and methods that increase a lubricating oil's resistance to oxidation.
Various compositions have been suggested as antioxidants in art areas other than the lubricating oil arts. For example, U.S. Pat. No. 3,010,937 discloses that thioacetals are useful antioxidizing agents in polymers, particularly, polyolefins. Linear and branched dialkyl thioformals are included as examples of thioacetals.
U.S. Pat. No. 4,735,980 discloses that methylene bis (alkylsulfides) behave as antioxidant synergists in rubber when combined with phenolic and amine type antioxidants.
Although these references teach the usefulness of the aforementioned compounds as antioxidants for stabilizing polymers such as rubber, the references neither teach nor suggest the synergistic combinations of this invention for use as lubricating oil antioxidants.
In the art of lubricating oils, U.S. Pat. No. 3,478,107 discloses that branched-chain alkyl formaldehyde mercaptals of the formula R'--S--CH.sub.2 --S--R", where R' and R" are independently branched-chain C.sub.3 --C.sub.4 alkyl radicals, are useful antiwear additives in lubricating oils. However, this reference neither discloses or suggests this invention's synergistic combination of dithioketals with phenolic or phosphite ester antioxidants. In fact, the surprising nature of this invention is not only demonstrated by the oxidation induction times reported herein, but also by the observation that the dithioketal synergistic effect is achieved with only select compounds possessing antioxidant characteristics.